Conventionally, sheet processing apparatuses that perform various types of inspections of sheets have been put to practical use. A sheet processing apparatus has an image reading apparatus that reads an image of a sheet. The sheet processing apparatus takes in sheets that are placed in a feed unit in such a manner that one sheet is taken in at a time, and conveys the sheets to the image reading apparatus. The sheets that are processed by the sheet processing apparatus may be banknotes, bills, or securities etc.
The image reading apparatus includes an illuminator and a sensor. The image reading apparatus irradiates a conveyed sheet with light, and the sensor detects optical features (characteristics) of that sheet from reflected light. The sheet processing apparatus identifies the sheet by comparing various preset parameters with the detected characteristics. Moreover, the image reading apparatus includes a background plate that has a reference color part (e.g., white part) for stably reading an image from a sheet. The image reading apparatus corrects the image in accordance with light from the reference color part.
In cases where a sheet is conveyed at a high speed, the sheet may be skewed or slid. Thus, the image reading apparatus detects an edge of the sheet during conveyance, detects skewing and sliding of the sheet from the detected edge, and corrects an image of the sheet based on the detected skewing and sliding.
In order to detect an edge of a sheet, the background plate of the image reading apparatus has a region in which a black region is disposed so as to prevent reflected light from entering the sensor. However, if a black region is disposed in the background plate, there is a problem in that the image of the sheet may be affected.